Catechism of the Catholic Church §2011 — excerpt
The charity of Christ is the source in us of all our merits before God. Grace, by uniting us to Christ in active love, ensures the supernatural quality of our acts and consequently their merit before God and before men.
This translation is under copyright, so only the cited words are quoted here. Follow the link for the full text. Translation: English translation of the Catechism of the Catholic Church, 2nd ed. (Libreria Editrice Vaticana), as posted at vatican.va.
Also cites: St. Therese of Lisieux, "Act of Offering" in Story of a Soul, tr. John Clarke (Washington Dc: ICS, 1981), 277